Surface effects vehicle having variable geometry lift fan

ABSTRACT

An improved surface effects vehicle having a variable geometry, double air inlet centrifugal fan for heave motion control. The fan provides rapid response to heave variation with minimal actuating power. The fan is mounted on the vehicle and disposed to direct air downwardly and into the plenum of the vehicle. The fan has a means for varying its volume rate of flow by changing the effective cross-section of the air passage therethrough.

United States Patent Luscher 1 June 17, 1975 [54] SURFACE EFFECTS VEHICLE HAVING 3,l8l,636 5/1965 Cockercll l80/ll8 3,208,543 9/l965 Crowley l80/l l6 X VARIABLE GEOMETRY LIFT FAN 3,458,007 7/1969 Todd l8U/l26 X [75] Inventor: Werner P. Luscher, El Dorado Hills. 3,590,939 7 1971 Grihangne 180/1 11;

Calif.

[73] Assignee: Aerojet-General Corporation, El Primary schonbefg Mome m Assistant E.rummer-Terrance L. Siemens Attorney, Agent, or Firm-John L. McGannon [22] Filed: Nov. [2, 1973 [21] Appl. No.: 415,082 [57] ABSTRACT An improved surface effects vehicle having a variable 52 us. (:1. 180/118; l80/126; 415/153 geometry, double air inlet centrifugal fan for heave 1511 1m. (:1. B60v 1/04 motion COMO!- The fan Provides rapid response to 5 Field of Search 180/] 16, 3 1,20 2 heave variation with minimal actuating power. The fan 4 93 15 7 is mounted on the vehicle and disposed to direct air downwardly and into the plenum of the vehicle. The [56] References Cited fan has a means for varying its volume rate of flow by UNITED STATES PATENTS changing the effective cross-section of the air passage I therethrough. 272,595 2/1883 Smith 4l5/98 l.604,328 10/1926 Walker 4l5/l58 19 Claims, 7 Drawing Figures 04 52 5 4 Cl I 94, )3 9s, 9s, 3

POWER VAR. cro. COMP. CONTROLLE COME V DEVICES SLEEVES r 10 0 PHASE SENSOR REF. 2 2 51cm 92 g VEH l C L E ACCELEROMETER M N W DISTURBANCES PATENTEDJUN 1 1 m5 3 a 89,775

SHEET 1 FIG.|

1 f 54 1 l0 /2e E 36 Q 40 42 2. 2's v 24 24 s 26" I2 II I0 10 22 1 F41 i8 %fi4r|220 I: 1 l I I l6. y; 53 r4 FIG-2 FIG.3

COMP lcoNIRoLLERH COMP 7 POWER u DEVICES SLEEVES 1 sensor: REF

' 22 SIGNAL 92\ S v m ACCELEROMETER E a FIG.7 A

DISTURBANCES SURFACE EFFECTS VEHICLE HAVING VARIABLE GEOMETRY LIFT FAN This invention relates to improvements in surface ef fects vehicles. such as ships and the like, and, more par ticularly. to a surface effects vehicle having heave attenuation means thereon.

BACKGROUND OF THE INVENTION The lift fan system of most surface effects ships operating over water has a number of sub-systems which are required for the ship to perform efficiently and with stable response characteristics. Among these subsystems is a heave motion control system to accommodate the vehicle for changes in pressure within its plenum. The present methods of heave motion control are inefficient and waste a considerable amount of power used to drive the lift fan, the fan which forces air under pressure into the plenum for sustaining the vehicle. Conventional heave control due to plenum pressure fluctuations is obtained through venting the plenum by the use ofpuff ports." Such venting is inefficient in that it does not minimize the heave accelerations to which the crew and occupants of the vehicle are subjected.

For long vehicle missions, the heave acceleration should be less than 0.10 gs for reason ofcrew comfort. Since the plenum pressures are proportional to heave acceleration, the fan system used on a surface effects ship must then tolerate dynamic pressure fluctuations of of the mean plenum chamber. This is accomplished by careful selection of the plenum pressureflow characteristic slope at the mean operating points. The slope requirement to meet the stated habitability limits are generally between 400 to l000 Ft /Sec/Lbs/Ft These slope and resulting flow swing requirements are very difficult to meet with present fixed geometry fan characteristics without entering the reverse slope (stall) region of the plenum pressure-flow characteristic at very low flow requirements. Since occasionally large pressure fluctuations must be accepted, these would surpass the fan shutoff head capability and fan back-venting. Thus, a stall can result which is structurally detrimental to the vehicle.

As the above mentioned slope increases, the flow range also increases and a reduction in average fan efficiency can result. The frequencies of the dynamic pressure fluctuations are a function of vehicle size, the vehicle natural frequency response. and the wave and counter frequencies, i.e., the vehicle velocity and the sea state. For a typical 2,000 ton surface effects ship, the dynamic response frequencies are less than 2 Hz.

SUMMARY OF THE INVENTION The present invention is directed to an improved surface effects vehicle having a variable geometry lift fan for use in providing heave attenuation to avoid the need for plenum venting at a considerable savings in fan power, such as a savings of to in power. The lift fan for the vehicle features rapid-response variable geometry controls which operate with minimal actuating power. The lift fan itself is preferably a double suction air inlet, centrifugal fan which has several advantages over single suction device designs including the following; smaller diameter, lighter weight, smaller volume, zero axial thrust, better vehicle integration capability due to its smaller size, easier to make and lower manufacturing costs due to its smaller size, fewer bearings, shafts, seals and couplings, and reversibility of the fan rotor.

Because of its variable geometry means, the lift fan of the surface effects vehicle of this invention can operate at a lower rotor speed since less power is required to maintain the proper air pressure in the vehicle plenum. This feature assures that the lift fan will generate less noise than conventional lift fans and can be made of less expensive materials because of the reduction in mechanical constraints thereon. Moreover, the lift fan can be relatively lightweight in construction and relatively small in size because of the power savings capable of being realized with its use. With conventional heave attenuators, a large space is required which mini mizes usable deck area of the vehicle. Also, the dual inlet of the lift fan assures that all unbalanced dynamic forces exerted on the fan itself will be cancelled. This feature is not contemplated with conventional lift fans since the latter have only steady state control rather than dynamic control.

The primary object of this invention is to provide an improved surface effects vehicle having a lift fan provided with variable geometry means so that the volume rate of flow of air through the fan and into the plenum of the vehicle can be varied for heave attenuation to thereby eliminate the need for venting the plenum as is now required conventionally.

A further object of this invention is to provide an improved lift fan for use in a number of different applications, including use as the lifting means for a vehicle of the type described, wherein the lift fan has improved variable geometry means in the form of a pair of concentric sleeves which vary the effective cross-section of the passage through the fan to permit immediate changes in the volume rate of flow of air therethrough with minimal disturbance within the fan itself.

Other objects of this invention will become apparent as the following specification progresses, reference being had to the accompanying drawings for illustrations of the invention.

In the drawings:

FIG. 1 is a fragmentary, cross-sectional view of the deck of a surface effects vehicle showing the variable geometry lift fan therefor;

FIGS. 2 and 3 are front and side elevational views, respectively, of the vehicle on a body of water;

FIG. 4 is a side elevational view of the lift fan on the vehicle;

FIG. 5 is a perspective view of a typical rotor for the lift fan;

FIG. 6 is a vertical section through the lift fan taken along line 66 of FIG. 4; and

FIG. 7 is a heave acceleration sensing system of the vehicle of the invention.

The surface effects vehicle forming the subject of this invention is broadly denoted by the numeral 10 and is illustrated in FIGS. 13. For purposes of illustration, the vehicle is a ship having a propulsion unit II, such as a water jet, and deck 12 provided with a pair of buoyant sides 14 which support the deck above the water surface 16. Front and rear skirts or walls I8 and 20 co-operate with sides 14 to present a closed plenum 22 below the deck, above water surface I6 and within the side, front and rear boundaries defined by sides 14 and walls 18 and 20. Air is directed under pressure into plenum from an improved lift fan 24 mounted on deck 12 so that its air outlet (FIG. 4) is in communication with plenum 22. The lift fan is powered by a power source 26 also on the deck and connected by a shaft 28 to the lift fan.

Lift fan 24 is of the double suction centrifugal type and is illustrated in FIGS. 1 and 4-6. To this end, lift fan 24 has a housing 29 provided with a pair of sides 30 and 32, a pair of air inlets 34 and 36 (FIGSv I and 6) through respective sides 30 and 32, and a rotor 38 having a central divider or plate 39 provided with a pair of opposed flat sides near its outer periphery. Rotor 38 rotates about an axis through shaft 28. The shaft is mounted by bearings 29 on the spaced legs 31 of a pair of spider assemblies and is coupled to power source 26 directly or through a gear box 44 for reducing the rotational speed of the shaftv Typically, the power source comprises a gas turbine engine.

Rotor 38 has a plurality of blades or vanes 40 on one flat side of plate 39 and a plurality of blades 42 on the opposite flat side of plate 39. The plates are in a fluid passage extending between respective air inlets 34 and 36 and air outlet 25 of fan 24. Thus, the blades cause a flow of air through such passage when rotor 38 rotates in one direction. As shown in FIG. 5, the blades are curved in the direction of rotation of the rotor, such direction being denoted by arrow 43 in FIG. 5. The radially innermost and outermost ends of each blade are denoted by the numerals 45 and 47 (FIG. 5).

Lift fan 24 has variable geometry means 50 (FIGS. 1 and 6) for causing variations in the volume rate of flow of air through the fan. This feature is used to compen sate for heave variations of vehicle 10 due to changes of volume of plenum 20, such as when the vehicle moves over water in which there is wave action or otherwise rough water.

Means 50 preferably includes a pair of cylindrical sleeves 52 and 54 provided adjacent to and surrounding each of the air inlets of lift fan 24, respectively, such sleeves being interconnected by a ring 55 and being concentric to shaft 42. Each pair of sleeves is shiftably carried by an annular, transversely C-shaped mount 56 carried by adjacent legs 31 (FIG. 6). The sleeves are slidably mounted on the inner surfaces of respective mounts so that the sleeves are movable axially of and relative to rotor 38.

A number of power devices 62 at uniformly spaced locations about each air inlet are provided to shift the adjacent pair of sleeves 52 and 54. Typically, power devices 62 ancl l20 apart. Each power device 62 has a reversible rod 64 which passes through an opening 66 in the adjacent mount 56 and is connected to the adjacent ring 55 for shifting the sleeves axially of the rotor when the power device is actuated. The power devices corresponding to each pair of sleeves are actuated simultaneously and power devices 62 on one side of rotor 38 may be operated simultaneously with those on the opposite side of the rotor, or may be operated without the power devices on the opposite side being operated.

When it is desired to operate lift fan 24 with the maximum volume rate of flow of air therethrough, sleeves 52 and 54 are in the full open positions of FIG. 6. When it is desired to decrease the volume rate of flow through the fan, each power device 62 is actuated (assuming that both pairs of sleeves move together), thereby displacing the sleeves, causing them to reduce the effective widths of respective rotor blades. This causes a reduction in the effective cross-section of the fluid passage through the fan, thereby reducing the volume rate of flow of air thcrethrough. To increase the volume rate of flow, the sleeves are moved outwardly by reversing the directions of movement of rods 64.

While means 50 has been described with respect to a centrifugal fan, such means can also be used with pumps and compressors in a number of different applications wherein such pumps and compressors operate in centrifugal flow, mixed flow or axial flow modes. In any case, the volume rate of fluid flow will be controlled by sleeve means to increase or decrease the cross-sectional area of the fluid flow passage.

While means 50 has been described as including a pair of concentric sleeves for each side of rotor 38, it is possible that a single sleeve could be used on each side of the rotor. Such sleeve preferably should be in the radially outermost sleeve position of FIG. 6 for op timal air flow through the fan.

In operation, vehicle 10 is advanced over the water by a suitable propelling means, such as a waterjet, (not shown mounted on deck 12. As it is advanced, the vehicle is sustained above the water by the air pressure within plenum 22. Such air pressure is created by the flow of air from lift fan 24 into the plenum. Power unit 26 which drives the lift fan is operated at constant power output since any air pressure changes are compensated for by the actuation of variable geometry means 50.

When it is desired to decrease the air pressure in the plenum, sleeves S2 and 54 are shifted inwardly of the fan to reduce the effective cross-section of the fluid passage through the fan. This causes a reduction in the volume rate of flow of air into the plenum, thereby reducing its air pressure. The ship thus settles somewhat to overcome, for instance, the effect of heaving due to rough water. To increase the fluid pressure of the plenum, the sleeves are shifted outwardly of the fan to increase the volume rate of air flow into the plenum. The inward and outward shifting of the sleeves can be done quickly so that the effects of heave accelerations on the ship are minimized.

It has been determined that the mean power requirements for lift fan 24 are approximately 15% to 20% less than the bleed control techniques used conventional surface effects ships of the 2,000 ton type. The signifi cance of this is not only considerable fuel and power savings but also additional overload capability since the capacity of the power source of the fan is not fully utilized and thus further increases the range of the vehicle. Thus, the variable geometry concept provides improved dynamic control for a surface effects ship over that conventionally available.

Besides the dynamic advantages of the variable geometry control of fan 24, certain steady state benefits can be obtained. The off-design performance, such as operation on calm seas, can be improved by operating the fan partially closed to improve fan performance as compared to speed control only. This can be done by optimizing the fan closure-speed relationship for various mean plenum pressures. The closure of the fan at low flow requirements also reduces the energy input during back-venting conditions; thus, it is a safeguard for structural integrity. Variable geometry control also permits fan design at higher head coefficients and, therefore, permits lower tip speeds of the fan blades.

One means of sensing and controlling the shifting of sleeves S2 and 54 to change the volume rate of flow through the fan is shown in FIG. 7 and utilizes a vehicle acceleration feedback system. The heave acceleration a, of the vehicle, i.e., the rate of change of velocity of 5 a predetermined location of the vehicle as it moves vertically, is the control variable which would tend to fluctuate under the influence of cushion disturbances in plenum 22. Such fluctuations would be sensed by an accelerometer 92 placed at the aforesaid predetermined location on the vehicle and providing an ac. feedback signal which would be compared with an input or a.c. reference signal a by means of a comparator 94 after the reference signal has been directed through a phase shifting network 95 to provide an adequate basis for comparison with a signal from accelerometer 92. The difference between the two signals would generate an error signal 0 which would be converted by a controller 96 to the desired fan variable geometry command position signalv This signal would be compared by a second comparator 98 with the signal representing the actual variable geometry position since a geometry sensor 100 is coupled in negative feedback relationship from power devices 62 to comparator 98. Thus, sensor 100 would detect the operative positions of the rods 64 of power devices 62. If an error signal a, is generated by comparator 98, power devices 62 would be actuated to shift rods 64 and thereby change the operating positions of sleeves 52 and 54. As the sleeve positions change, a greater or lesser volume of air is directed into the plenum to cushion the vehicle due to upward and downward movements thereof.

The control system of FIG. 7 is designed to assure that means 50 will be dynamically stable and will possess the frequency response characteristics which will allow control response over the frequency means of acceleration disturbances of vehicle 10. The system compensates for vehicle heave motion, duct and plenum dynamics and variable geometry fan characteristics.

I claim:

1. A surface effects vehicle comprising: a body hav ing means defining an open-bottom plenum adapted to be substantially closed when the body is disposed on a surface; an actuatable, centrifugal type lift fan carried by the body and having a fluid passage adapted to direct air under pressure into said plenum to form an air cushion to support the same above said surface and to permit movement of the body over the surface; means coupled with said lift fan and movable across said fluid passage for causing a change in the volume rate of flow of air therethrough when the lift fan is actuated; and means coupled with said lift fan for actuating the same.

2. A vehicle as set forth in claim 1, wherein said plenum forming means includes a pair of spaced, buoyant sides, a front skirt, and a rear skirt spaced from the front skirt, said skirts spanning the distance between said sides.

3. A vehicle as set forth in claim 1, wherein is provided a propulsion unit mounted on said body for mov' ing the latter over said surface.

4. A vehicle as set forth in claim 1, wherein said lift fan has a fluid passage therethrough, said causing means including structure movable relative to said lift fan and across said passage to change the effective cross section thereof, and actuatable means coupled with said structure for moving the same relative to said lift fan.

5. A vehicle as set forth in claim 4, wherein said structure includes a cylindrical sleeve.

6. A vehicle as set forth in claim 4, wherein said structure includes a pair of concentric sleeves,

7. A vehicle as set forth in claim 4, wherein said lift fan has a rotor, said structure including a sleeve movable axially of the rotor.

8. A vehicle as set forth in claim 7, wherein said rotor has a plurality of spaced blades thereon, there being an air inlet for the lift fan adjacent to the axis of the rotor. said structure including a sleeve movable axially of the rotor to vary the effective widths of the rotor blade.

9. A vehicle as set forth in claim 8, wherein each rotor blade has a radially innermost end and a radially outermost end, the sleeve being movable into operative positions adjacent to the radially outermost ends of the rotor blades.

10. A vehicle as set forth in claim 1, wherein said lift fan has a rotor provided with a pair of opposed sides and a plurality of spaced blades on each side thereof, respectively, said lift fan having a pair of air inlets on respective sides of the rotor, said causing means including a sleeve for each air inlet, respectively, the sleeves being movable axially of said rotor and disposed to vary the widths of respective blades, whereby the effective cross-section of the fluid passage through the fan is changed to thereby cause a change in the volume rate of flow of air through the lift fan.

11. A vehicle as set forth in claim 1, wherein said body has a deck, said lift fan being disposed on the deck and having an air outlet communicating with said plenum through the deck.

12. A vehicle as set forth in claim 1, wherein is provided means coupled with said causing means for actuating the same as a function of the vertical acceleration of said body at a predetermined location thereon relative to said surface.

13. A surface effects vehicle as set forth in claim 1, wherein said actuating means comprises a gas turbine engine.

14. A surface effects vehicle as set forth in claim 1, wherein said body comprises a surface effects ship movable over the water and having a weight of at least 2,000 tons.

15. A surface effects ship comprising: a buoyant hull having a deck, a pair of spaced sides, a front skirt, and a rear skirt, the deck, the sides and the skirts defining a closed air-receiving plenum when the hull is on a body of water; a lift fan carried by the deck and having a pair of air inlets, an air outlet spaced from said air inlets, and a fluid passage extending between the air inlets and the air outlet, said air outlet being in fluid communication with said plenum, said lift fan having a rotor, the axis of the rotor passing through the air inlets, said rotor having a pair of opposed sides and a plurality of spaced blades on each side thereof, respectively; a sleeve for each side of the rotor, respectively, the sleeves surrounding respective air inlets and being movable axially of the rotor to thereby vary the effective widths of respective blades, whereby the effective cross section of said fluid passage is changed to cause a change in the volume rate of flow of air therethrough', means coupled with said hull for sensing the movement of a predetermined location thereon as the hull moves relative to a surface; means coupled with the sleeves for moving the same as a function of the sensing of said hull movement; and means coupled with the rotor for rotating the same in a direction to cause a flow of air through said fluid passage and into said plenum to form an air cushion for supporting said hull on the water.

16. A method of controlling the operation of a surface effects vehicle having a plenum closed by a surface therebelow comprising: directing air under pressure along a spiral path continuously into said plenum to provide an air cushion therein to thereby support said body above said surface and to allow said body to move over the same; moving the body over the surface; and changing the cross section of the path to vary the volume rate of flow of air into said plenum as a function of the heave acceleration of a predetermined location thereof.

17. A method as set forth in claim 16, wherein said varying step includes sensing said acceleration. developing an error signal in response to the sensing step and changing said volume rate of flow as a function of said error signal.

l8. A method as set forth in claim 16, wherein said path has a circular portion, said changing step includ ing blocking an annular part of said path portion,

19. A surface effects vehicle comprising: a body having means defining an open-bottom plenum adapted to be substantially closed when the body is disposed on a surface; an actuatable lift fan carried by the body and adapted to direct air under pressure into said plenum to form an air cushion to support the same above said surface and to permit movement of the body over the surface; means coupled with said lift fan for causing a change in the volume rate of flow of air therethrough when the lift fan is actuated; means coupled with said lift fan for actuating the same; and actuatable reciprocatory actuating device responsive to an error signal; and means responsive to the vertical acceleration of a predetermined location of said body and coupled with said actuating device for generating an error signal representative of the magnitude of said acceleration with respect to a predetermined reference. 

1. A surface effects vehicle comprising: a body having means defining an open-bottom plenum adapted to be substantially closed when the body is disposed on a surface; an actuatable, centrifugal type lift fan carried by the body and having a fluid passage adapted to direct air under pressure into said plenum to form an air cushion to support the same above said surface and to permit movement of the body over the surface; means coupled with said lift fan and movable across said fluid passage for causing a change in the volume rate of flow of air therethrough when the lift fan is actuated; and means coupled with said lift fan for actuating the same.
 2. A vehicle as set forth in claim 1, wherein said plenum forming means includes a pair of spaced, buoyant sides, a front skirt, and a rear skirt spaced from the front skirt, said skirts spanning the distance between said sides.
 3. A vehicle as set forth in claim 1, wherein is provided a propulsion unit mounted on said body for moving the latter over said surface.
 4. A vehicle as set forth in claim 1, wherein said lift fan has a fluid passage therethrough, said causing means including structure movable relative to said lift fan and across said passage to change the effective cross section thereof, and actuatable means coupled with said structure for moving the same relative to said lift fan.
 5. A vehicle as set forth in claim 4, wherein said structure includes a cylindrical sleeve.
 6. A vehicle as set forth in claim 4, wherein said structure includes a pair of concentric sleeves.
 7. A vehicle as set forth in claim 4, wherein said lift fan has a rotor, said structure including a sleeve movable axially of the rotor.
 8. A vehicle as set forth in claim 7, wherein said rotor has a plurality of spaced blades thereon, there being an air inlet for the lift fan adjacent to the axis of the rotor, said structure including a sleeve movable axially of the rotor to vary the effective widths of the rotor blade.
 9. A vehicle as set forth in claim 8, wherein each rotor blade has a radially innermost end and a radially outermost end, the sleeve being movable into operative positions adjacent to the radially outermost ends of the rotor blades.
 10. A vehicle as set forth in claim 1, wherein said lift fan has a rotor provided with a pair of opposed sides and a plurality of spaced blades on each side thereof, respectively, said lift fan having a pair of air inlets on respective sides of the rotor, said causing means including a sleeve for each air inlet, respectively, the sleeves being movable axially of said rotor and disposed to vary the widths of respective blades, whereby the effective cross-section of the fluid passage through the fan is changed to thereby cause a change in the volume rate of flow of air through the lift fan.
 11. A vehicle as set forth in claim 1, wherein said body has a deck, said lift fan being disposed on the deck and having an air outlet communicating with said plenum through the deck.
 12. A vehicle as set forth in claim 1, wherein is provided means coupled with said causing means for actuating the same As a function of the vertical acceleration of said body at a predetermined location thereon relative to said surface.
 13. A surface effects vehicle as set forth in claim 1, wherein said actuating means comprises a gas turbine engine.
 14. A surface effects vehicle as set forth in claim 1, wherein said body comprises a surface effects ship movable over the water and having a weight of at least 2,000 tons.
 15. A surface effects ship comprising: a buoyant hull having a deck, a pair of spaced sides, a front skirt, and a rear skirt, the deck, the sides and the skirts defining a closed air-receiving plenum when the hull is on a body of water; a lift fan carried by the deck and having a pair of air inlets, an air outlet spaced from said air inlets, and a fluid passage extending between the air inlets and the air outlet, said air outlet being in fluid communication with said plenum, said lift fan having a rotor, the axis of the rotor passing through the air inlets, said rotor having a pair of opposed sides and a plurality of spaced blades on each side thereof, respectively; a sleeve for each side of the rotor, respectively, the sleeves surrounding respective air inlets and being movable axially of the rotor to thereby vary the effective widths of respective blades, whereby the effective cross section of said fluid passage is changed to cause a change in the volume rate of flow of air therethrough; means coupled with said hull for sensing the movement of a predetermined location thereon as the hull moves relative to a surface; means coupled with the sleeves for moving the same as a function of the sensing of said hull movement; and means coupled with the rotor for rotating the same in a direction to cause a flow of air through said fluid passage and into said plenum to form an air cushion for supporting said hull on the water.
 16. A method of controlling the operation of a surface effects vehicle having a plenum closed by a surface therebelow comprising: directing air under pressure along a spiral path continuously into said plenum to provide an air cushion therein to thereby support said body above said surface and to allow said body to move over the same; moving the body over the surface; and changing the cross section of the path to vary the volume rate of flow of air into said plenum as a function of the heave acceleration of a predetermined location thereof.
 17. A method as set forth in claim 16, wherein said varying step includes sensing said acceleration, developing an error signal in response to the sensing step, and changing said volume rate of flow as a function of said error signal.
 18. A method as set forth in claim 16, wherein said path has a circular portion, said changing step including blocking an annular part of said path portion.
 19. A surface effects vehicle comprising: a body having means defining an open-bottom plenum adapted to be substantially closed when the body is disposed on a surface; an actuatable lift fan carried by the body and adapted to direct air under pressure into said plenum to form an air cushion to support the same above said surface and to permit movement of the body over the surface; means coupled with said lift fan for causing a change in the volume rate of flow of air therethrough when the lift fan is actuated; means coupled with said lift fan for actuating the same; and actuatable reciprocatory actuating device responsive to an error signal; and means responsive to the vertical acceleration of a predetermined location of said body and coupled with said actuating device for generating an error signal representative of the magnitude of said acceleration with respect to a predetermined reference. 